1. Field of the Invention
The present invention relates to a process for hydroprocessing a hydrotreated liquid distillate stream to produce a stream exceptionally low in sulfur as well as aromatics. A hydrotreated distillate stream is further hydrotreated in a co-current reaction zone wherein the reaction product is passed to a separation zone thereby producing a vapor phase product and a liquid phase product. The resulting vapor phase product is collected overhead and the resulting liquid phase product is passed to an aromatics saturation stage and passed through a bed of hydrogenation catalyst countercurrent to the flow of hydrogen-containing treat gas.
2. Background of the Invention
Environmental and regulatory initiatives are requiring ever lower levels of both sulfur and aromatics in distillate fuels. For example, proposed sulfur limits for distillate fuels to be marketed in the European Union for the year 2005 is 50 wppm or less. There are also regulations that will require lower levels of total aromatics in hydrocarbons and, more specifically, to lower levels of multiring aromatics found in distillate fuels and heavier hydrocarbon products. Further, the maximum allowable aromatics level for U.S. on-road diesel, CARB reference diesel, and Swedish Class I diesel are 35, 10 and 5 vol. %, respectively. Further, the CARB and Swedish Class I diesel fuels allow no more than 1.4 and 0.02 vol. % polyaromatics, respectively. Consequently, much work is presently being done in the hydrotreating art to meet these regulations.
Hydrotreating, or in the case of sulfur removal, hydrodesulfurization, is well known in the art and usually requires treating the petroleum streams with hydrogen in the presence of a supported catalyst at hydrotreating conditions. The catalyst is typically comprised of a Group VI metal with one or more Group VIII metals as promoters on a refractory support. Hydrotreating catalysts that are particularly suitable for hydrodesulfurization, as well as hydrodenitrogenation, generally contain molybdenum or tungsten on alumina promoted with a metal such as cobalt, nickel, iron, or a combination thereof. Cobalt promoted molybdenum on alumina catalysts are most widely used when the limiting specifications are hydrodesulfurization, while nickel promoted molybdenum on alumina catalysts are the most widely used for hydrodenitrogenation and partial aromatic saturation.
Much work is also being done to develop more active catalysts and improved reaction vessel designs in order to meet the demand for more effective hydroprocessing processes. Various improved hardware configurations have been suggested. One such configuration is a countercurrent design wherein the feedstock flows downwardly through successive catalyst beds counter to upflowing treat gas, which is typically a hydrogen containing treat-gas. The downstream catalyst beds, relative to the flow of feed, can contain high performance, but otherwise more sulfur sensitive catalysts because the upflowing treat gas carries away heteroatom components, such as H2S and NH3, that are deleterious to sulfur and nitrogen sensitive catalysts.
Other process configurations include the use of multiple reaction stages, either in a single reaction vessel, or in separate reaction vessels. More sulfur sensitive catalysts can be used in the downstream stages as the level of heteroatom components becomes successively lower. European Patent Application 93200165.4 teaches a two-stage hydrotreating process performed in a single reaction vessel, but there is no suggestion of a unique stripping arrangement for the liquid reaction stream from each reaction stage.
Two types of process schemes are commonly employed to achieve substantial hydrodesulfurization (HDS)/ aromatics saturation (ASAT) of distillate fuels and both are operated at relatively high pressures. One is a single stage process using Ni/Mo or Ni/W sulfide catalysts operating at pressures in excess of 800 psig. To achieve high levels of saturation pressures in excess of 2,000 psig are required. The other process scheme is a two stage process in which the feed is first processed over a Co/Mo, Ni/Mo or Ni/W sulfide catalyst at moderate pressure to reduce heteroatom levels while little aromatics saturation is observed. After the first stage, the product stream is stripped to remove H2S, NH3 and light hydrocarbons. The first stage product is then reacted over a Group VIII metal hydrogenation catalyst at elevated pressure to achieve aromatics saturation. Such two stage processes are typically operated between 600 and 1,500 psig.
In light of the above, there is a need for improved desulfurization/aromatic saturation process for treating feedstreams so that they can meet the ever stricter environmental regulations.
In accordance with the present invention there is provided a multi-stage process for reducing the sulfur and aromatics content of a distillate boiling range feedstock containing greater than 1,000 wppm sulfur, which process comprises:
a) reacting said feedstock in a first reaction stage in the presence of a hydrogen-containing treat gas, said first stage containing one or more reaction zones operated at hydrodesulfurization conditions, including temperatures from about 200xc2x0 C. to about 300xc2x0 C., and pressures from about 100 to 1,000 psig, wherein each reaction zone contains a bed of hydrodesulfurization catalyst comprised of at least one Group VI metal and at least one Group VIII metal on a refractory support, thereby resulting in a partially desulfurized feedstock containing from about 100 wppm to about, but not including, 1,000 wppm sulfur;
b) reacting the partially desulfurized feedstock from said first reaction stage in a second reaction stage in the presence of a hydrogen-containing treat gas cascaded from, or partially cascaded from, the third stage herein, said first second stage containing one or more reaction zones operated at hydrodesulfurization conditions, including temperatures from about 200xc2x0 C. to about 300xc2x0 C., and pressures from about 100 to 1,000 psig, wherein each reaction zone contains a bed of hydrotreating catalyst comprised of at least one Group VI metal and at least one Group VIII metal on a refractory support, and wherein said second reaction stage is operated in the substantial absence of recycle hydrogen-containing treat gas;
c) passing the resulting desulfurizied feedstock to a separation zone wherein a vapor phase stream and a liquid phase stream are produced;
d) collecting said vapor phase stream overhead; and
e) passing said liquid phase stream to a third reaction stage in the presence of a hydrogen-containing treat gas, said reaction stage containing one or more reaction zones operated at aromatics saturation conditions including temperatures of about 2000 to 400xc2x0 C. and pressures of about 100 to 1,000 psig, wherein each reaction zone contains a bed of aromatics saturation catalyst, and wherein said hydrogen-containing treat gas is passed through said reaction stage countercurrent to the flow of said liquid phase stream, wherein said third reaction stage is operated in the substantial absence of recycle hydrogen-containing treat gas.
In a prefered embodiment of the present invention, the liquid phase stream, before it passes through said second reaction stage is contacted with a vapor to strip dissolved gases from the liquid phase.